A single crystal scintillator of thallium-doped sodium iodide [NaI(Tl)], which has been widely used as a radiation detector, costs relatively little to produce and gives high light output in response to radiation. It is therefore used, instead of a Geiger counter, for many applications including high energy physics studies and exploring oil reservoirs. This scintillator has many defects, however. The radiation detection efficiency is low due to the low density of NaI(Tl) which requires a large detector. The radiation counting ability is lower due to a long fluorescence and afterglow decay time which results in blurring or merging of fluorescence signals. Handling at high temperatures and high humidity is difficult due to deliquescence.
In order to overcome these defects, a bismuth germanate Bi.sub.4 Ge.sub.3 O.sub.12 (BGO) scintillator has been used. The BGO scintillator has a high density and radiation absorption rate, but its defects include low light output and long fluorescence decay time.
Also, in order to overcome these defects a single crystal scintillator of cerium doped gadolinium silicate [Gd.sub.2 SiO.sub.5 :Ce] (GSO) has been proposed in Japanese Patent Examined Publication No. 62-8472 corresponding to U.S. Pat. No. 4,647,781. The GSO scintillator, however, has a peak luminescence wavelength of 430 nm, which is outside the maximum 380 nm to 420 nm spectral sensitivity range of the photomultiplier tube with a bi-alkali photocathode, commonly used in combination with these scintillators. Thus the light-electrical signal conversion is not optimum.
U.S. Pat. No. 4,883,956 to Melcher et al. proposes a GSO single crystal scintillator for use in an apparatus for exploring underground formations. Due to the properties of GSO, e.g., relatively small fluorescence output, relatively long decay time (60 ns), and relatively long luminescence wavelength, however, the performance of this apparatus is not very suitable.
In order to have the single crystal scintillator in positron CT or various radiation measuring instruments detect sufficient radiation and to make the apparatus smaller, the radiation absorption rate of the single crystal scintillator should be high. Furthermore, since the performance of the apparatus depends on the extent of the fluorescence output, this output must be large to improve the S/N ratio (fluorescence output signal/noise ratio) of images in an image processing apparatus. Moreover, the decay time must be reduced to improve the counting rate performance which is influenced by pulse pile-up. In addition, to use the fluorescence output effectively, it is better to match the wavelength of the photomultiplier tube which shows the maximum spectral sensitivity with the luminescence wavelength of the single crystal scintillator. It is therefore necessary to improve the luminescence wavelength of the known single crystal scintillators.